Standardized test mixtures are needed to determine the effectiveness and efficiency of distillation columns and their internals. Despite the wide range of different test mixtures and the variations in their physical properties, no standard test mixture is available that contributes viscosities above 1.5 mPa s at typical distillation conditions. To investigate the impact of viscosity on separation effectiveness and efficiency and validate models for higher viscosities, a new test mixture is proposed. On the basis of a screening, the binary mixture 2-methyl-2-butanol/2-methyl-1-propanol was chosen, showing liquid phase viscosities up to 5 mPa s and satisfying the requirements for separation efficiency tests. For this mixture, vapor-liquid equilibria were measured from 2 to 95 kPa. A slight negative deviation from ideal behavior is accounted for using the nonrandom two-liquid model. Furthermore, for both pure substances as well as their mixtures liquid densities were determined from 293.15 to 363.15 K and viscosities in a temperature range from 273.15 to 333.15 K. Nonidealities were modeled with a Redlich-Kister approach. Thus, vital information for the assessment of separation efficiency with a new test mixture is given and subsequent studies on the influence of viscosity on fluid dynamics and separation efficiency in distillation columns are enabled.